Translation guide
The English word 'century' refers to a period of 100 years. In Japanese, the most common way to express this is with the counter 世紀 (seiki), but there are also other expressions for specific contexts.
century (100 years)
Use with a number: 20世紀 (20th century).
To refer to a specific century in history or a 100-year span.
The 20th century was an era of great change.
This building was built in the 18th century.
Literally '100 years'. Used when emphasizing the duration of 100 years rather than a historical period.
This tree has lived for over a century.
To refer to a batsman scoring 100 runs in cricket.
A loanword from English used specifically in cricket contexts.
He scored a century in the match.
To refer to a military unit of about 100 soldiers in ancient Rome.
A loanword from Latin 'centuria', used in historical contexts.
A century of the Roman army was commanded by a centurion.
To say 'the Xth century', use [number] + 世紀. For example, 21世紀 (にじゅういっせいき) means '21st century'. Note that Japanese uses the same ordinal number as English (21st, not 20th).
The word 百年 (ひゃくねん) means '100 years', but for 'centennial' (100th anniversary), use 百周年 (ひゃくしゅうねん) or 百年祭 (ひゃくねんさい).
The 21st century began in 2001.
It's been a century since that event.